The invention relates to a method for strengthening the base of a hollow plastics blow-moulded body, particularly a plastics bottle intended to withstand high internal pressures such as aerosol containers and the like.
The usual cylindrical wall portion of such plastic containers is subjected predominantly to tensile stresses which plastics materials can withstand, especially when they are orientated biaxially by low temperature blow-moulding. At the transition between the cylindrical wall and the base, which must have a supporting surface so that the container can stand sufficiently firmly, a bending stress is encountered which, however, plastics materials cannot readily withstand because of their comparatively low elasticities. This leads to breakage or deformation of the containers, especially after a long period of storage or with high storage temperatures.
In order to remove these disadvantages, the transition from the cylindrical container wall to the supporting surface of the base can be modified or adapted as shown in German Pat. Specification No. 1,607,895, but it is essential to maintain a minimum diameter for the base support surface in order than an empty container shall have sufficient stability. In addition, one can achieve a concentration of material near the base of the container by regulating the thickness of the wall of the preformed blank from which the container is produced during extrusion or by suitably shaping the preformed blank during injection.
Also, at least three standing feet can be moulded on to a convexly curved base of a container as shown in German Gebrauchsmuster No. 1,950,569, or a support ring of a different material may be attached to the base, the latter measure being indeed sufficient but requiring additional parts and manufacturing steps.
However, attempts to produce a pressure-proof container using a minimum quantity of plastics material have lead to difficulties when forming the base. Thus, the measures described above, if the amount of material employed is to be kept to a minimum, are not satisfactory when producing containers for liquids having a high carbonic acid content or high internal pressures and large volumes of, for example, more than half a liter. The only exception to this is the use of a convexly curved base with an additional support ring. But this arrangement is more expensive and more complicated.
As already stated, for the base of a pressure-proof container which must possess sufficient stability without additional support, the transition from the cylindrical wall to the area within the supporting surface is vital. The radius of curvature of the area within the supporting surface should be as small as possible so that the bending moment in this region can be kept small. In addition, the diameter of the supporting surface must be as large as possible for a light bottle or other container to have sufficient stability. Any inward curving of the base must be such as to withstand the internal pressures to which it is likely to be subjected. None of the blow-moulding processes hitherto proposed can fulfill all the above-stated conditions.
In an extrusion blowing process it is difficult to form a small-radius, concavely curved base portion. Because of the grip of the bottom tools, the permissible concavity allowed in the base is limited. Although a small-radius concavity can be produced relatively easily this can be achieved only by substantial reduction of the wall thickness in this area of the base. The strength of the base is thus considerably reduced. Moreover, because of the longitudinal weld normally used in such processes, the containers cannot be subjected to high internal pressures.
Injection blow-moulding or similar processes, which employ a preformed blank which is already closed at one end, generally permit strengthening by biaxial orientation of the plastics material, do not have a longitudinal weld and are therefore employed in preference for the production of pressure-resistant containers. Thermoplastic materials which can be blown are substantially tougher in the thermoelastic temperature zone than in the thermoplastic zone. Consequently, the formation of impressed contours and also small-radius concavities is limited when there is strong curvatuve of the bottom. However, if pressures are applied which bring about impressed contour moulding, the thickness of the wall in the drawn-out area is considerably reduced.
Normally, at least with large volume containers, the only solution is to produce containers having convex bases and to attach support rings to them. The object of the invention is accordingly to manufacture a pressure-resistant container in a simple and convenient manner, with a small outley in a material.